Pluggin the text-ovelay causes some slowdowns and might need a colorspace
converter. We now only check the text-overlay property whne goint to ready and
plug or unplug the text-overlay at that time. If the property changes during
playing, its not switched immediately.
When we get bus messages for setting the xid, we have the real sink element
instance as message_src. No need to proxy the xoverlay iface therefore
(autovideosink does not do it either). Also we don't need to rewrite the
message src of all messages from that sink.
